A so-called ALD (Atomic Layer Deposition) method has been widely utilized for film formation processes of semiconductor manufacturing because of its good thermal history and step coverage.
Thus, this ALD method supplies steam flow of two or more raw material gases and liquid raw material gases alternatively to the inside of a process chamber, in order to form a film due to a chemical reaction on a surface of a wafer, or the like. That is, by employing the ALD method, it is possible to highly accurately form a film thickness corresponding to one atomic layer in a so-called “one sequence.”
A film formation of titanium nitride (TiN), using titanium tetrachloride (TiCl4) and ammonia (NH3) as precursors, is an important process in semiconductor manufacturing. Furthermore, the accuracy of control of a flow rate of supplying titanium tetrachloride (TiCl4) has great effects on the film thickness of the titanium nitride and on the quality of the titanium nitride film.
Therefore, various techniques with respect to the supply of raw material gases, such as titanium tetrachloride (TiCl4), have been conventionally developed. For example, in a conventional raw material gas supply apparatus (Japanese Patent No. 4605790) of FIG. 4, a carrier gas G1′ is supplied to the inside of a source tank 25 through a pressure regulator 22 and a mass flow controller 23 from a carrier gas source 21, and a mixed gas G0′ of steam G2′ of a liquid raw material 24 and the carrier gas G1′ is supplied to the inside of a process chamber 29 through a pressure control valve CV and an opening/closing valve V1, to control the supply of the gas G0′ to the process chamber 29 by controlling the opening or closing of the pressure control valve CV and the opening/closing valve V1.
In addition, in FIG. 4, reference numeral 27 denotes an automatic pressure regulating device for regulating the pressure of the inside of the source tank 25, and the automatic pressure regulating device computes the tank internal pressure from the pressure in a pipe passage L (as measured by pressure sensor PO) and a detection value of a temperature (as measured by temperature sensor TO), and controls the opening or closing of the pressure control valve CV in a direction in which a difference with a set pressure input from a terminal 28 becomes zero, thereby keeping the source tank internal pressure at a set value. Reference numeral 27a denotes a controller within automatic pressure regulating device 27. Furthermore, reference numeral 26 denotes a constant temperature heating unit, reference symbol 30 denotes a heater, reference symbol 31 denotes a wafer, Gn′ denotes another raw material gas, Vn denotes an opening/closing valve for the other raw material gas Gn′.
In the above-described raw material gas supply apparatus of FIG. 4, first, the pressure of the carrier gas G1′, which is supplied to the inside of the source tank 25, is set to a predetermined pressure value PG1 (as measured by the corresponding pressure sensor) by the pressure regulator 22, and its supply flow rate is set to a predetermined flow rate value by the thermal type mass flow control system (mass flow controller) 23. Moreover, the portion of the source tank 25, and the like, are heated and kept at a high temperature of about 150° C. The supply quantity of the carrier gas G1′, the temperature of the source tank 25, and the internal pressure of the source tank 25 (the pressure of the mixed gas G0′) are respectively kept at set values, thereby supplying the mixed gas G0′ of a constant mixture ratio with a constant flow rate to the process chamber 29 through the pressure control valve CV while highly accurately controlling at a predetermined flow rate value, which is proportional to a set flow rate by the thermal type mass flow control system 23, and the mixed gas G0′ is supplied to the process chamber 29 by opening the opening/closing valve V1.
FIG. 5 shows another example of this type of raw material gas supply apparatus. A liquid raw material gas (TiCl4) in the source tank 25 is evaporated by a bubbling action of the carrier gas G1′, a mixed volume G0 of the carrier gas G1′, raw material gas steam G2′, and raw material gas particles associated with the carrier gas, are made to flow into a vaporizer 35, and a vaporized mixed gas G0′ is supplied to a valve opening/closing mechanism 34 through buffer tanks 33, and a mixed gas G0′ of a predetermined quantity is supplied to the inside of the chamber 29 by opening/closing control (on/off control) of the valve V1.
In addition, in FIG. 5, respectively, the liquid raw material gas (TiCl4) 24 in the source tank 25 is heated to about 100° C. (steam pressure of 269 Torr), and the vaporizer 35 is heated to about 200° C., and the respective buffer chambers 33 (whose internal volumes are about 500 to 1000 cc) are heated to about 170° C., and the valve opening/closing mechanism 34 is heated to about 200° C. Furthermore, the flow rate of supplying the mixed gas (TiCl4+carrier gas) G0′ is about 20 sccm (standard cubic centimeters per minute), and the supply pressures of argon (Ar) and ammonia (NH3) are 0.15 PaG, and the supply flow rates thereof are respectively about 10 SLM (Standard Liter per Minute). Moreover, the internal volume of the process chamber 29 is 500 to 1000 cc, and the internal pressure thereof is kept to 1 Torr or less.
At the time of supplying the raw material gases to the chamber 29, the respective raw material gases stored in the respective buffer tanks 33 at predetermined internal pressures are supplied in sequence by predetermined quantities by sequentially turning on/off the opening/closing valves V1 to Vn in the valve opening/closing mechanism 34 at predetermined time intervals (for example, an opening time is about 0.2 seconds and a closing time is about 0.93 seconds in the case of TiCl4), in order to perform one cycle of film formation.
In the gas supply apparatus shown in FIG. 4, because the pressure of the space portion in the source tank 25 (the pressure of the mixed gas G0′) is kept to a set value by the automatic pressure regulating device 27 in the source tank, it is possible to supply the raw material gas G0′ of a predetermined quantity to the valve opening/closing mechanism 34 (i.e., opening/closing valve V1 in particular) while highly accurately controlling the flow rate thereof even without using the buffer tanks 33. This is because the gas supply apparatus of FIG. 4 does not employ buffer tanks.
Furthermore, in the gas supply apparatus shown in FIG. 5 as well, because the buffer tanks 33 are used, there are no pressure fluctuations in the respective raw material gases G0′, GAr, and GNH3 to be supplied. This makes it possible to supply the respective raw material gases G0′, GAr, and GNH3 at predetermined flow rates to the inside of the chamber 29 through the valve opening/closing mechanism 34, which exerts an excellent effect.
However, there remain many unsolved problems in the conventional gas supply apparatuses shown in FIG. 4 and FIG. 5. First, in the gas supply apparatuses shown in FIG. 4 and FIG. 5, because the steam G2′ of the liquid raw material gas 24 is supplied as a raw material gas to the process chamber 29 by use of the carrier gas G1′, it is not possible to directly supply only the steam G2′ of the liquid raw material gas 24 to the process chamber 29. That is, it is not possible to supply only the stream G2′ of the liquid raw material gas 24 to the process chamber 29 without the use of a carrier gas. As a result, there is a problem that it takes a lot of time and effort for managing the concentration of the raw material gas G2′ in the mixed gas G0′, which makes it difficult to highly accurately control the supply quantity of the raw material gas G2′.
Furthermore, in the gas supply apparatus shown in FIG. 4, there are problems listed as follows: (A) because the expensive thermal type mass flow control system 23 is used, it is difficult to reduce the manufacturing cost for raw material vaporizing and supplying apparatuses and, additionally, it is necessary to highly accurately control the carrier gas supply pressure to the thermal type mass flow control system 23, which leads to an increase in the equipment cost for the pressure regulator 22; (B) it is not possible to directly control a flow rate of the mixed gas G0′ by the thermal type mass flow control system 23; (C) because the apparatus adopts the bubbling method, it is difficult to stably supply raw material steam in the case of a solid raw material, or a raw material at low steam pressure, which often makes it unstable to supply a mixed gas to the process chamber; (D) the concentration of the raw material steam G2′ in the mixed gas G0′ significantly fluctuates according to a fluctuation in the raw material liquid level in the source tank, which makes it difficult to control the concentration of the raw material steam G2′; (E) because the carrier gas flow rate on the inlet side and the mixed gas flow rate (total flow rate) on the outlet side are different from each other, highly accurate flow control of the mixed gas flow rate is difficult; and (F) it is not easy to highly accurately control the internal pressure of the source tank and, as a result, it is not easy to regulate a raw material concentration directly relating to the partial pressure of the raw material steam in the mixed gas in the tank, and the like.
Moreover, in the gas supply apparatus in FIG. 5, in addition to the problems of (A) to (F) in the gas supply apparatus in FIG. 4, as listed above, there is a problem that, because the apparatus is configured to control a supply quantity of the raw material gas G0′ by adjusting its opening/closing switching time by use of the opening/closing valve V1 provided in the switching mechanism 34 as a pulse-driven valve, not only is achieving highly accurate flow rate control difficult, but also it takes a lot of time and effort for maintenance and management of the opening/closing valve V1. Moreover, in the gas supply apparatus of FIG. 5, the buffer chambers 33 are required in order to stabilize the supply pressure of the raw material gas G0′, which makes it impossible to downsize the apparatus.